El Planeta Tierra Clase 2

8/12/2019 El Planeta Tierra Clase 2

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1 Planet EarthFor hundreds of thousands of years, humans have learned to survive by ada pting to the

natural rhythms and cycles of the planet on which we live. U nderstanding our environment

has always been essential. B ut from our beginnings, we have been a ble to perceive only our

immediate landscape, usually only a few square kilometers. Our perspective for o bserving

and understanding Earth a nd its changes was very limitedat b est, a va ntage point on a

high hill or mounta intop.

Since the 1960s, how ever, we ha ve seen our planet a s it really is, a tiny b lue sphere sus-

pended in the blackness of empty space, as this photograph of t he International Space

Sta tion hovering over E arths cloud-covered oceans illustrates so drama tically. Astro naut

photographs and satellite images have revealed the exquisite beauty of E arth, a planet

washed in water and surrounded by a thin atmosphere with swirling white clouds.Viewed

from space, our planetour homeis a system of moving gas, liquid, and solids with nu-

merous interconnected and interdependent components.

Our need to understand E arth a s a system is one of the rea sons the different views

from space are so valuable and so exciting. B ut the exploration of space did more than


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increase our understanding of E arth. We have landed on the Moo n, mapped the surfaces

of Mercury, Venus, and M ars, and surveyed the diverse landscapes of the moons of Jupiter,

Saturn, U ranus, and Neptune. E very object in the solar system contains part of a record of

planetary origin and evolution tha t helps us understand our own planet.

B ack on Ea rth, we have also extended our explorations to the vast unknown of the

ocean floo r. We have mapped its landfo rms and structure, gaining insight into its origin and

history. We now know t hat t he rocks below t he ocean floor a re completely different from

those below the surface of the cont inents. We also have peered into E arths depths using

indirect methods. We have tra ced the paths of eart hqua ke-generated seismic wa ves,

measured the amount of heat that escapes from inside Ea rth, and recorded the pulse of the

magnetic field. Co nsequently, we ha ve discovered how E arths interior is churning slowly

and how such movements affect processes at the very surface of the planet.

With these new perspectives of our planet, we must develop a n all-encompassing view of

how E arth operates as a constantly changing dynamic system. In this chapter we start to do

this by comparing and contrasting other planets with Earth.We also describe the major

features of continents and ocean basins and view Earths internal structureall features

that ma ke Planet E arth unique in the solar system.

Photograph courtesy of NA SA .


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INTRODUCTION TO GEOLOGY

G eology is the science of E arth. It concerns all of E arth: its origin, its

history, its materials, its processes, and the dyna mics of how it changes.

Geologyis an incredibly fascinating subject. It is concerned w ith such diverse phe-nomena a s volcanoes and glaciers, rivers and bea ches, earthq uakes and landslides,

and even the history of life. G eology is a study about w hat happened in the past

and what is happening nowa study that increases our understanding of nature

and o ur place in it.

Yet geology d oes much more than satisfy intellectual curiosity. We are at a point

in human history when E arth scientists have a responsibility to help solve some of

societys most pressing problems. These include finding sites fo r safe disposal of

radioa ctive waste and toxic chemicals, determining responsible land use for an

expanding population, and providing safe, plentiful water supplies. G eology is

being called upon to guide civil engineers in planning buildings, highway s, da ms,

harbors, and cana ls. G eology helps us recognize how devasta tion caused by natural

hazards, such as landslides, earthquakes, floods, and beach erosion, can be avoid-

ed or mitigated. Another driving force in our attempt to understand E arth is thediscovery of natural resources. All E arth materials, including water, soils, miner-

als, fossil fuels, and building materials, are geologic and are discovered, exploit-

ed, and ma naged with the aid o f geologic science.

G eologists also offer key informat ion about the entire global system, especially

past climate change and likely causes and effects of future climate modification.

Perhaps, in the end, more fully comprehending nature is as important as the dis-

covery of o il fields and mineral depo sits.

Let us begin by exploring why E arth is unique among the planetary bo dies of

the solar system. We will then examine some of its important cha racteristics: its

size, composition, atmosphere, hydrosphere, and the structure of its interior.

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1. A comparison of Earth with other inner planets reveals the distinguishingcharacteristics of our planet and shows what makes it unique.

2. E arths at mosphere is a thin shell of gas surrounding the planet. It is a fluidin constant motion. Other planets have atmospheres, but E arths is unique be-

cause it is 78% nitrogen and 21% oxygen.

3. The hydrosphere is anot her feature tha t makes E arth unique. Water movesin a great, endless cycle from the ocean to the atmo sphere, over the land sur-

face, and back to the sea again.4. The biosphere exists because of wa ter. Altho ugh it is small compared w ith

other layers of E arth, it is a major geologic force operating at the surface.

5. Continents and ocean basins are the largest-scale surface features of Earth.6. The continents have three major components: (a) a ncient shields,

(b) stable platforms, and (c) belts of folded mountains. E ach reveals the

mobility of E arths crust.

7. The major structural features of the ocea n floor are: (a) the ocea nic ridges,(b) the vast abyssal floor, (c) long, narrow, and incredibly deep trenches, (d)

seamounts, and (e) continental margins.

8. E arth is a differentiated planet, with its materials segregated into layers ac-cording to density. The internal la yers classified by composition are (a ) crust,

(b) ma ntle,a nd (c) core. The ma jor internal layers classified by physical prop-

erties are (a) lithosphere, (b) asthenosphere, (c) mesosphere, (d) outer core,and (e) inner core. Material within each of t hese units is in motion, making

E arth a changing, dynamic planet.

MAJ OR CONCEPTS


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P l a n e t E a r t h

EARTH COMPARED WITH OTHER PLANETS

Among the inner planets (Mercury, Venus, E arth, the Moon, and Ma rs),

E arth is unique because of its size and distance from the Sun. It is large

enough to develop and retain an atmosphere and a hydrosphere.

Temperature ranges are mod erate, such that w at er can exist on its surface

as liquid, solid, and gas.

The Solar System

A map of the solar system (Figure 1.1) shows the Sun and the major planetary

bod ies. This is Ea rths cosmic home, the place of its origin and development. All of

the planets in the solar system were created a t the same time a nd from the same

general material. The massive Sun, a star that generates heat b y nuclear fusion,

is the center of the system. B ecause of the Suns vast gravitational influence, all

of the planets orbit around it. As seen from above their north poles, the planets

move counterclockwise about the Sun in slightly elliptical orbits. Moreover, all

orbit in the same plane as the Suns equator, except Pluto (note the different

inclination of its orbit).

The diagra m of the sola r system in Figure 1.1 is not, of course, to scale. The or-

bits are distorted, and the sizes of the planetary bo dies are greatly exaggerated a nd

shown in perspective. In reality,t he orbits are extremely large compared with the

planets sizes. A simple analogy ma y help convey the size and structure of the

solar system. If the Sun were the size of an orange, E arth would be roughly the

size of a grain of sand o rbiting 9 m (30 ft) awa y. Jupiter would be the size of a pea

revolving 60 m (200 ft) aw ay. P luto wo uld be like a gra in of silt 10 city blocks

aw ay. The nearest star wo uld be the size of a nother orange more than 1600 km

(1000 mi) a wa y.

U ntil recently, the planets and their moons were mute astronomical bodies, only

small specks viewed in a telescope. B ut toda y, they are new w orlds as real as our

own, because we have landed on their surfaces and studied them with remotely con-

trolled probes. One of the most fundamental facts revealed by our exploration of

the solar system is that the sizes and compositions of the planets vary systemati-cally with distance from the Sun (Figure 1.2).The inner planets (Figure 1.3) in-clude Mercury and the planetlike Moon, with their cratered surfaces;Venus, with

its extremely hot, thick atmosphere of ca rbon dioxide and numerous volcanoes;

E arth, with cool blue seas, swirling clouds and multicolored lands; and Ma rs, with

huge canyons, giant extinct volcanoes, frigid polar ice caps, and long, dry river beds.

The la rge outer planetsJupiter, Saturn, U ranus, and Neptuneare giant balls ofgas, with ma jestic rings and dozens of sma ll satellites composed mostly of ice. The

most distant planet, P luto, is small and similar to these icy moons. Indeed, wa ter ice

is the most common rock in the outer solar system. The densityof a planet ormoon revea ls these drama tic differences in composition. (D ensity is a measure of

ma ss per unit volume: g/cm3). For example, the densities of the rocky inner plan-

ets a re q uite high (over 3 g/cm3) compared to the gas- and ice-rich outer planets

which ha ve densities less tha n ab out 1.6 g/cm3

.Our best evidence tells us that E arth formed, along with the rest of the solar

system, abo ut 4.6 billion years ago. Nonetheless, only the inner planets are even

vaguely like E arth. The compositions (dominated by d ense solids with high melt-

ing points) of the inner planets make them radically different from the outer

planets, made of low -temperature ices as well as gas. Although the inner planets

are roughly of the same general size, mass, and composition, they vary w idely in

wa ys that a re striking and important to us as living creatures. Why is Ea rth so dif-

ferent from its neighbors? Why does it alone have ab undant liquid w ater, a dy-

namic crust, an oxygen-rich atmo sphere, and perhaps most unique, that intricate

web o f life, the biosphere?

How do the inner planets differ fromthe outer planets?

Tour of the Solar System


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Saturn

Jupiter

Venus

Moon

Earth

Mercury

Uranus

Neptune

Pluto

Mars

FIGURE 1.1 Our solar systemconsists of one star, a family of nine planets (almost 70 moons discovered so far), thousands of asteroids, andbillions of meteoroids and comets (not shown here). The inner planets (Mercury, Venus, Ea rth with its Moon, and Ma rs) are composed mostly of

rocky materials. The outer planets (Jupiter, Saturn, U ranus, and Neptune) are much larger, are composed mostly of gas and liquid, and have no

solid surfaces. Pluto a nd C haron a nd the satellites in the outer solar system are composed mostly of water ice. Some are so cold (230 C) tha t they

have methane ice or nitrogen ice at their surfaces.

All planetary bo dies in the solar system are important in the study of E arth because their chemical compositions, surface features, and o ther

characteristics show how planets evolve.They provide important insight into the forces that shaped our planets history.

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Earth

From a planeta ry perspective, E arth is a small blue planet ba thed in a film of white

clouds and liquid wa ter (Figure 1.3). In this remarkable view, we see Ea rth mo-

tionless, frozen in a moment of time, but there is much more action shown here

than yo u might imagine. The blue water a nd swirling white clouds dominat e the

scene and underline the importance of moving water in the E arth system. H uge

qua ntities of wa ter are in constant motion, in the sea, in the air (as invisible vapor

and condensed as clouds), and on land. You can see several complete cyclonic storms

spiraling over thousands of sq uare kilometers, pumping vast amo unts of wa ter into

the atmo sphere.When this wat er becomes precipitation on land, it flows back to the

sea in great river systems that erode a nd sculpt the surface.

La rge parts of Africa and A ntarctica a re visible in this view. The major climac-

tic zones of our planet are clearly delineated. For example, the great Saha ra D esert

is visible at the top of the scene, extending across North Africa a nd into a djacentSaudi Arabia. Much of the vast tropical rain forest of central Africa is seen be-

neath the discontinuous cloud cover. Also, large portions of the south polar ice

cap a re clearly visible.

E arth is just the right distance from the Sun fo r its temperature to let water

exist as a liquid, a solid, and a gas.Water in any of those forms is part of the hy-

drosphere. If E arth were closer to the Sun, our oceans would evaporate; if it

were farther from the Sun, our oceans would freeze solid. H owever, there is

plenty of liquid water on E arth, and it is liquid water, as much as anything else,

that ma kes E arth unique among the planets of the solar system. H eated by the

Sun, wat er moves on E arth in great cycles. It evaporates from the huge oceans

into the atmo sphere, precipitat es over the landscape, collects in river systems,

and ult imately flows back to the oceans. As a result , E arths surface stays

young, being constantly changed by wa ter and eroded into intricate systemsof river valleys. This dynamism is in remarkable contra st to o ther planetary b od-

ies, the surfaces of w hich are do minated by t he craters of ancient meteorite im-

pacts (Figure 1.3).

The presence of w ater a s a liquid on E arths surface througho ut its long histo-

ry also enabled life to evolve. And life, strange as it may seem, has profoundly

changed the composition of E arths atmosphere. H ere is the mechanism: Photo -

synthesis by countless plants removes large quantities of carbon dioxide from

the atmosphere. As part of this process, the plants exhale oxygen. In a ddition,

many forms of marine life remove carbon dioxide from seawater to make their

shells, which later fall to the seafloor and f orm limestone.

How has space photography changedour view of Earth and its geologicsystems?

FIGURE 1.2 The planets of the solar systemvary in size and composition with distance from the Sun. The inner planets are small and ro cky,whereas the outer planets are much larger and composed mostly of hy drogen and helium.

1 10Astronomical units from Sun

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto


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Venusis often considered Ea rths twin because of its similar size and

density, but the two planets are not identical.This image of Venus shows its

cloudy atmosphere partially stripped awa y to reveal a rad ar map of t he solid

surface made by a n orbiting satellite.Venus has high plateaus, folded mountain

belts, many volcanoes,a nd relatively smooth volcanic plains,but it ha s no water

and few meteorite impact craters.

D iamet er 12,100 km. D ensity 5.25 g/cm3

Earthis a delicate blue ball wra pped in filmy white clouds. The wa ter and

swirling clouds that do minate E arths surface underline the importance of

wat er in E arths systems.The cold polar regions are buried with ice,a nd the

warm tropics are speckled with clouds and greenery. The rocks of the high

continents are strongly deformed and older than t he rocks of the ocean

basins. Ea rth has active volcanoes, a dynamic interior, and no large

impact craters are visible on its surface.

D iam ete r 12,800 km D ensit y 5.55 g/cm3

Mars is much smaller than E arth a nd Venus but has ma ny fascinating geologic featuresevidence that its

surface has been dyna mic in the past. Three huge extinct volcanoes, one more than 28 km high, can be

seen in the left part of this image.A n enormous canyon extends across the entire hemispherea distance

roughly eq ual to tha t from New York to San Francisco. These features reveal that todays windy, desert

Martian surface has been d ynamic in the past, but ancient meteorite impact craters (visible in the upper right

part of t he image) have not b een completely obliterated by younger events.

D iam ete r 6800 km D ensit y 3.9 g/cm3

The Moon has two contrasting provinces: bright, densely cratered highlands and dark, smooth lava plains.We know from rock

samples brought back by the Apolloastrona uts that the da rk smooth plains are ancient floods of lava that filled many large meteorite

impact craters and spread out o ver the surrounding area. The volcanic activity thus occurred after the forma tion of the hea vily

cratered terrain, but wa s not sufficient to obliterate a ll of the impact craters. Today the Moon is a geologically quiet body w ith no

atmosphere or liquid wa ter. D iameter 3500 km D ensity 3.3 g/cm3

Mercury is similar to the Mo on, with a surface dominated by

ancient impact crat ers and younger smooth plains presumably

made from f loods of lava. Like the Moon, Mercury lacksan a tmosphere and hydrosphere.

D iam ete r 4900 km D ensity 5.44 g/cm3

Mars

Venus

Mercury

Moon

FIGURE 1.3 The surfaces of the inner planets,shown at the same scale, provide insight into planetary dynamics. (Courtesy of NA SA/JPL /Caltech)

Earth

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Another characteristic of E arth is that it is dynamic. Its interior and surface

continually change as a result of its internal heat. In marked contrast , manyother planetary bod ies have changed little since they fo rmed because they are

no longer hot inside. Most of E arths heat comes from natural radioactivity.Thebreakdown of three elementspotassium, uranium, and t horiumis the prin-

cipal source of this heat. Once generated, this heat flows to the surface and is

lost to space. Another source of heat has been inherited from the formation of

the planets. H eat w as generated in each of the planets by the infall of countless

meteorites to form a larger and larger planet. This accretionary heatmay havemelted the early planets, including E arth. La rger planets have more internal

heat and retain it longer than smaller planets.

E arths internal heat creates slow movements within the planet. Its rigid outer

layer (the lithosphere) breaks into huge fragments, or plates, that move. Over bil-

lions of years, these moving plates have created ocea n basins and continents. The

heat-driven internal movement also has d eformed E arths solid o uter layers, cre-

ating earthq uakes, mountain belts, and volcanic activity. Thus, E arth has alwa ys

been a dynamic planet, continuously changing as a result of its internal heat and

the circulation of its surface water.

Look a gain at the view of E arth from space (Figure 1.3). Of particular inter-

est in this view is the rift system of E ast Africa. The continent is slowly being

ripped apart a long this extensive fracture system. Where this great rift separatesthe Arabian P eninsula from Africa, it has filled with wa ter,fo rming the R ed Sea.

The rift extends from there southw ard across most of the continent (it is mostly

obscured by clouds in the equat orial region). Some animals that evolved in the

East African rift valleys spread from there and learned to live in all of the var-

ied landscapes of the planet. This wa s their first home, but they ha ve since walked

on the Moon.

The Other Inner Planets

In stark contrast to the dynamic Ea rth, some of the other inner planets are com-

pletely inactive and unchanging. For example, the Moo n and M ercury (Figure 1.3)

are pockmarked with thousands of craters that record the birth of the planets

ab out 4.55 billion years ago. This wa s a period when planeta ry bodies swept upwha t remained of the cosmic debris that f ormed the Sun and its planets. As the de-

bris struck each bod y, impact cratersformed.The Moon a nd Mercury are so small that they were unable to generate a nd re-

ta in enough internal heat to sustain prolonged geologic activity.They rapidly cooled

and lost the a bility to make volcanoes. Their smooth lava plains are a ncient by

comparison with those on E arth. Co nsequently, their surfaces have changed little

in billions of yea rs. They retain ma ny meteorite impact cra ters formed during the

birth of the solar system. Neither planet has a hydrosphere or an a tmosphere to

modify them. Thus, these small planets remain as fossils of the early stages in

planetary development.The footprints left on the Moon by the Apol loastronauts

will remain fresh and unaltered fo r millions of years. Mars is larger and ha s more

internal heat a nd a thin atmosphere (Figure 1.3). Its originally cratered surface

has been modified by volcanic eruptions, huge rifts, and erosion by w ind and, in itsdistant past, running wa ter.Today, Mars is too cold and the atmo spheric pressure

too low fo r water to exist as a liquid. La rge polar ice caps mark both poles. In many

wa ys, Mars is a frozen wasteland with a nearly immobile crust. Co nsequently, its an-

cient impact craters were never completely obliterated.

Venus is larger still and ha s more internal energy,which moves the crust and con-

tinually reshapes its surfaces (Figure 1.3). Venus is only slightly smaller tha n E arth

and closer to the Sun. A thick carbo n dioxide-rich atmosphere holds in the solar

energy that reaches the surface and makes the temperature rise high enough to

melt lead (aro und 500 C). The atmo spheric pressure is 90 times that on E arth. U n-

like the smaller planets, Venus has no heavily cratered area s. Its a ncient impact

What can the surface features tell us

about planetary dynamics?



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10 C h a p t e r 1

craters must have been destroyed by deformation o r by burial below lava flows.

Its surface is appa rently young. B ecause of its large size, it has cooled quite slowly,

so that volcanoes may even be active today. On the other hand, no evidence of

wat er has been found on Venus; it has no oceans, no rivers, no ice caps, and only

a very little water vapor. Only E arth has large amounts of liquid water that ha ve

influenced its development thro ughout history.

This, then, is P lanet E arth in its cosmic settingonly a pale blue dot in space,

part of a fa mily of planets and moons that revolve about the Sun. It is a minor

planet bound to a n ordinary star in the outskirts of one galaxy amo ng billions. Yet,from a human perspective, it is a va st and complex system that has evolved over

billions of years, a home we a re just beginning to understand. Learning about E arth

and the forces that change itthe intellectual journey upon which you are about

to emba rkis a journey we hope you will never forget. Our study of the diverse

compositions and conditions of the planets should remind us of the delicate bal-

ance that allows us to exist at a ll.A re we intelligent enough to understand how o ur

world f unctions as a planet a nd to live wisely within those limits?

EARTHS OUTERMOST LAYERS

The outermost layers of Eart h are the atmo sphere, hydrosphere, and

biosphere. Their dynamics a re especially spectacular w hen seen from space.

Views of Earth from space like the one in Figure 1.3 reveal many features that

make E arth unique, and they provide insight into our planet s history of change. The

atmo sphere is the thin, gaseous envelope that surrounds Ea rth. The hydrosphere,

the planets discontinuous wa ter layer, is seen in the vast oceans. E ven parts of the

biospherethe organic realm, which includes all of E arths living thingscan be

seen from space, such as the dark green tropical forest of equato rial Af rica. The

lithospherethe outer, solid part of E arthis visible in continents and islands.

One of the unique features of E arth is that each of the planets major realms is

in constant motion and continual change. The atmosphere and the hydrosphere

move in dramatic and obvious ways. Movement, growth, and change in the bio-

sphere can be readily appreciatedpeople are part of it. B ut E arths seemingly im-mobile lithosphere is also in motion, and it has been so throughout most of the

planets history.

The Atmosphere

Perha ps Ea rths most conspicuous features, as seen from space, are the atmosphereand its brilliant white swirling clouds (Figure 1.3). Altho ugh this envelope of ga s

forms an insignificantly small fractio n of the planets mass (less tha n 0.01%), it

is particularly significant because it moves easily and is constantly intera cting with

the ocean and land. It plays a part in the evolution of most features of the land-

scape and is essential for life. On t he scale of the illustrat ion in Figure 1.3, most of

the atmo sphere wo uld be concentrated in a layer a s thin as the ink with which the

photo is printed.The at mospheres circulation pa tterns a re clearly seen in Figure 1.3 by the shape

and orientation of the clouds. At first glance, the patterns may a ppear confusing,

but upon close examination we find that they are well organized. If w e ignore the

details of local weather systems, the global atmospheric circulation becomes ap-

parent. Solar heat, the driving force of atmospheric circulation, is greatest in the

equatorial regions. The heat causes water in the oceans to evaporate, and t he heat

makes the moist a ir less dense, causing it to rise. The warm, humid a ir forms an

equatorial belt of spotty clouds, bordered on the north and south by zones that

are cloud-free, where a ir descends. To the north a nd south, cyclonic storm systems

develop where warm air from low latitudes confronts cold air around the poles.


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P l a n e t E a r t h 1

Our a tmosphere is unique in the solar system. It is composed of 78% nitrogen,

21% oxygen, and minor amounts of other gases, such as carbon dioxide (only

0.035%) and wa ter vapor. The earliest atmo sphere wa s much different. It wa s

essentially oxygen-free and consisted largely of carbon dioxide and water vapor.

The present carbon dioxide-poor atmosphere developed as soon as limestone

began to fo rm in the oceans, tying up the carbon dioxide. Oxygen was added to the

atmosphere later, when plants evolved. As a result of photosynthesis, plants

extracted carb on dioxide from t he primitive atmo sphere and expelled oxygen into

it. Thus, the oxygen in the atmosphere is and w as produced by life.

The Hydrosphere

The hydrosphere is the total mass of water on the surface of our planet. Watercovers about 71% of the surface. Ab out 98% of this wat er is in the oceans. Only

2% is in streams, lakes, groundwater, and glaciers. Thus, it is for good reason that

E arth has been called the water planet. It has been estimated that if all the ir-

regularities of E arths surface were smoot hed out to form a perfect sphere, a glob-

al ocean would cover Earth to a depth of 2.25 km.

Again, it is this great ma ss of wa ter that ma kes Earth unique.Water permitted

life to evolve and flourish; every inhabitant on E arth is directly or indirectly con-

trolled by it. All of E arths weather patterns, climate,ra infall, and even the amount

of carbon dioxide in the atmosphere are influenced by the water in the oceans.The hydrosphere is in constant mo tion; wa ter evaporates from the oceans and

moves through the atmosphere, precipitating as rain and snow, and returning to the

sea in rivers, glaciers, and groundwater. As water moves over Earths surface, it

erodes and tra nsports weathered rock mat erial and deposits it.These actions con-

stantly modify E arths landscape. Many o f E arths distinctive surface features are

formed by action of the hydrosphere.

The Biosphere

The biosphere is the part of E arth w here life exists. It includes the forests, grass-lands, and fa miliar animals of the land, together with the numerous creatures

that inhab it the sea and a tmosphere. Microorganisms such as bacteria a re too small

to be seen, but they are proba bly the most common form of life in the biosphere.As a terrestrial covering, the biosphere is discontinuous and irregular; it is an in-

terwoven web of life existing within and reacting with the a tmosphere, hydros-

phere, and lithosphere. It consists of more than 1.6 million described species and

perhaps as many a s 3 million more not y et described. E ach species lives within its

ow n limited environmenta l setting (Figure 1.4).

Almost the entire biosphere exists in a narrow zone extending from t he depth to

which sunlight penetrates the oceans (ab out 200 m) to the snow line in the tropical

and subtropical mountain ranges (abo ut 6000 m a bove sea level).A t the scale of the

photograph in Figure 1.3, the biosphereall of the know n life in the solar system

would be in a thin layer no thicker than the paper on which the image is printed.

Certainly one of the most interesting questions about the biosphere concerns

the number and va riety of organisms that compose it. Surprisingly, the truth is that

no one knows t he answer. D espite more than 250 years of systematic research, es-timates of the total number of plant and animal species vary from 3 million to

more tha n 30 million. O f this number, only 1.6 million species have been record -

ed. The diversity is stranger than you may think. Insects account for more than

one-half of a ll known species, wherea s there are only 4000 species of mamma ls, or

ab out 0.025% of a ll species. Ob servation shows tha t there a re more species of

small anima ls than o f large o nes. The smallest living creat uresthose invisible to

the unaided eye, such as protozoa, bacteria, and virusescontribute greatly to the

variety o f species. The biosphere is a truly rema rkable part o f E arth s systems.

The main factors controlling the distribution of life on our planet are temper-

ature, pressure, and chemistry of the local environment. Ho wever, the range of

How are Earths atmosphere andhydrosphere different from those onother planets?

What is the biosphere? How does itaffect Earth dynamics?


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environmental cond itions in which life is possible is truly amazing, especially the

range o f environments in w hich microorganisms can exist (Figure 1.4B ).

Although the biosphere is small compared with Earths other major layers (at-

mosphere, hydrosphere, and lithosphere), it has been a major geologic force. E s-

sentially all of the present atmosphere has been produced by the chemical activi-

ty of t he biosphere. The composition of the ocea ns is similarly af fected by living

things; many marine organisms extract calcium carbonate from seawa ter to

make their shells and ha rd part s. When the orga nisms die, their shells settle to the

seafloor and accumulate a s beds of limestone. In a ddition, the biosphere formedall of E arths coal, oil, and natural gas. Thus, much of the rock in E arths crust

_originated in some wa y from bio logical activity.

10,000

8,000

6,000

4,000

2,000

0

2,000

4,000

6,000

8,000

10,000

Elevation(m

eters)

Isolated communities atridge, energy from

heat and chemical reactions

Sea levelMost of biosphere occurs

within this zone

Bacteria down toseveral thousand meters

Highest mountainsAirborne bacteria and

some birds Upper limit of land animalsUpper limit of most plants

Upper limit of human habitation

Upper limit of agriculture

Scattered bottom-living animalsat the greatest depths reached

by underwater cameras

Sea level

(B) Most o f the biosphere exists within a very thin zo ne from 100 m below sea level to a bout 2000 m ab ove sea level.

(A)This map of the biosphere was produced from data derived from satellite sensors. La nd vegetation increases from tan to y ellow to green to b lack.

Escalating concentrations of ocean phytoplankton are shown by colors ranging from purple to red. Phyt oplankton are microscopic plants that live in the

surface layer of the ocea n and form the founda tion of the marine food chain. Note the particularly high concentrations of phytoplankton in polar waters

(red and yellow) and the very low concentrations (purple and b lue) in the mid-latitudes. (Courtesy of SeaWiFS Proj ect, NA SA/G oddard Space Fli ght Center

and Orbi mage)

Ocean plankton

Low High

Land vegetation

Low High

FIGURE 1.4 These global views of Earths biosphereemphasize that life is widespread a nd has become a powerful geologic force.


12/26


Why is it important to distinguishbetween the physical and chemical

layers of Earths interior?

A historical record of the biosphere is preserved, sometimes in remarkable

detail, by fossils that occur in rocks. Indeed, the number of living species today

represents less than 10% of t he number o f species that have existed since life first

developed on Ea rth.

EARTHS INTERNAL STRUCTURE

The solid materials of E arth a re separated into layers according to composi-tion and mechanical properties. From outside in, the compositional layers

are (1) crust, (2) mantle, and (3) core. La yers based on physical properties

are (1) lithosphere, (2) asthenosphere, (3) mesosphere, (4) outer core, and

(5) inner core.

Studies of earthquake waves, meteorites that fall to E arth, magnetic fields, and

other physical properties show tha t E arth s interior consists of a series of shells of

different compositions and mechanical properties. E arth is called a differentiatedplanetbecause of this separation into layers. How did E arth become differentiated?First, recall that the density of liquid wa ter is 1 g/cm3. The density of most ro cks at

the surface is a bout three times as grea t, just under 3 g/cm3. B ut the overall density

of E art h is abo ut 5.5 g/cm3. Clearly, E arth consists of internal layers of increasingdensity towa rd the center. The internal layers were produced as different materi-

als rose or sank so that the least-dense materials were at the surface a nd the most

dense were in the center of the planet. Thus, gravity is the motive force behind

Earths differentiated structure.

In the discussion below we ta ke you on a brief tour to the very center of E arth,

which lies at a depth of a bout 6400 km. Chemical properties define one set of lay-

ers, and mechanical behavior defines a different set. Figure 1.5 shows the layers

based on chemical properties on the left and those based on mechanical proper-

ties on the right. An understanding of both types of layers is vital.

Internal Structure Based on Chemical Composition

G eologists use the term crust for t he outermost compositional la yer (Figure 1.5,left). The base of the crust heralds a definite change in the proportions of the va r-

ious elements that compose the rock but not a strong change in its mechanical be-

havior or physical properties.

Moreover, the crust of the continents is distinctly different from the crust be-neath the ocean basins(Figure 1.6). C ontinenta l crust is much thicker (as much as75 km), is composed o f less-dense granitic rock (abo ut 2.7 g/cm3), is strongly

deformed, and includes the planets oldest rocks (billions of years in age). B y

contrast, the oceanic crust is only about 8 km thick, is composed of denser vol-

canic ro ck called b asa lt (about 3.0 g/cm3), is comparat ively undeformed by folding,

and is geologically young (200 million yea rs or less in age). These differences be-

tween the continental a nd oceanic crusts, as you shall see, are fundamental to

understanding E arth.

The next major compositional layer of E arth, the mantle,surrounds or coversthe core (Figure 1.5, left). This zone is ab out 2900 km thick and constit utes the

great bulk of E arth (82% of its volume and 68% of its mass).The mantle is com-

posed of silicate rocks (compounds of silicon and o xygen) that a lso contain ab un-

dant iron and ma gnesium. Fragments of the mantle have been brought to the

surface by volcanic eruptions. B ecause of the pressure of overlying rocks, the

ma ntles density increa ses with dept h from a bo ut 3.2 g/cm3 in its upper part t o

nearly 5 g/cm3 near its contact w ith the core.

Earths core is a central mass abo ut 7000 km in diameter. Its density increaseswit h depth b ut avera ges about 10.8 g/cm3. The core ma kes up only 16% of E arths

volume, but, because of its high density, it accounts for 32% of Ea rths mass.


13/26

14 C h a p t e r 1

Indirect evidence indicates tha t the core is mostly metallic iron, making it distinctly

different from the silicate material of the mantle.

Internal Structure Based on Physical Properties

The mechanical (or physical) properties of a material tell us how it responds to

force, how wea k or strong it is, and w hether it is a liquid or a solid. The solid, strong,

and rigid outer layer of a planet is the lithosphere( rock sphere ). The lithosphere

includes the crust and the uppermost part of the mant le (Figure 1.5, right). E arthslithosphere varies great ly in thickness, from a s little a s 10 km in some oceanic area s

to a s much as 300 km in some continental a reas. Figure 1.6 show s how the major

internal layers of E arth are related.

Within the upper mantle, there is a ma jor zone where temperature and pres-

sure are just right so that part o f the material melts, or nearly melts. U nder these

conditions, rocks lose much of their strength and b ecome soft and plastic and f low

slowly. This zone of ea sily defo rmed ma ntle is known as the asthenosphere(weaksphere). The bounda ry between the a sthenosphere and t he overlying lithosphere

is mechanically distinct but do es not correspond to a f undamental change in chem-

ical composition. The bounda ry is simply a major change in the rocks mechani-

cal pro perties.

The rock below the asthenosphere is stronger and more rigid than in the as-

thenosphere. It is so b ecause the high pressure at this depth off sets the effect ofhigh temperature, forcing the rock to be stronger than the overlying astheno-

sphere. The region betw een the a sthenosphere and the core is the mesosphere( middle sphere ).

Earths core marks a change in both chemical composition and mechanical

properties. On the ba sis of mechanical behavior alone, the core has two distinct

parts: a solid inner core and a liquid outer core.The outer core ha s a thicknessof a bout 2270 km compared with the much smaller inner core, with a radius of

only ab out 1200 km. The core is extremely hot, and hea t loss from the core and the

rotat ion of E arth proba bly cause the liquid outer core to flow.This circulation gen-

erates E arths magnetic field.

Lithosphere(Rigid)

Asthenosphere(Plastic)

Mesosphere(Solid)

Outer Core(Liquid)

InnerCore

(Solid)

Core(Iron)

Mantle(Silicates)

Crust Lithosphere

Asthenosphere

Mesosphere

2900 km

6370 km

5150 km

Crust(Silicates)

Mantle

8 to 75 km 0

100 to 300 km

350 to 500 km

Layers based on physical propertiesLayers based on chemical properties

FIGURE 1.5 The internal structure ofEarthconsists of la yers of d ifferent

composition and layers of different physical

propert ies.The left side shows the layering

based o n chemical composition. These

consist of a crust, mantle, and core.

The right side shows the layering based

on physical properties such as rigidity,

plasticity, and w hether it is solid or liquid.

(Note that the tw o divisions, chemical and

physical, do not coincide except at the core

mantle bo undary.)

What layers of Earth are most signifi-cant to planetary dynamics?


14/26


MAJOR FEATURES OF THE CONTINENTS

Continents consist of three major structural components: (1) shields;

(2) stable platforms; and (3) folded mo untain belts. Co ntinental crust is

less dense, thicker, older, and more deformed tha n oceanic crust.

If E arth ha d neither an atmo sphere nor a hydrosphere, two principal regions would

stand as its dominant features: ocean basins and continents. The ocean basins,

which occupy abo ut two-thirds of E arths surface, have a remarkable topography,

most of which originated from extensive volcanic activity and E arth movementsthat continue toda y. The continents rise abo ve the ocean ba sins as large platfo rms.

The ocean w at ers more than fill the ocean b asins and rise high enough to flood a

large part of t he continents. The present shoreline, so important to us geographi-

cally and so carefully mapped, has no simple relation to the structural boundary

between continents and ocean basins.

In our da ily lives, the position of the o cean shoreline is very important. B ut

from a geologic viewpoint, the elevation of the continents with respect to the

ocean f loor is much more significant than the position of the shore. The differ-

ence in elevation of continents and ocean b asins reflects their fundamental dif-

ference in composition and density. Co ntinental granitic rocks are less dense

(a bo ut 2.7 g/cm3) tha n the b asaltic rocks of the ocea n ba sins (about 3.0 g/cm3).

That is, a given volume of cont inental rock weighs less than the sa me volume of

oceanic ro ck.This difference causes the continental crust to be more buoyantto rise higherthan the denser oceanic crust in much the same way that icecubes float in a glass of wa ter because ice is less dense than w ater. Moreover, the

rocks of the continent al crust are older (some a s old as 4.0 billion years old) tha n

the rocks of the o ceanic crust.

The elevation and a rea of the continents and o cean ba sins now have b een

mapped with precision. These data can be summarized in various forms. Figure

1.7 shows that the a verage elevation of the continents is 0.8 km ab ove sea level,a nd

the average elevation of the seafloor (depth of the ocean) is about 3.7 km below

sea level. Only a relat ively small percentage of E arth s surface rises significantly

above the average elevation of the continents or drops below the average depth

60 km

Oceanic crust

Oceanic ridge

Lithosphere

Asthenosphere

Trench

70 km Continental crust

Foldedmountain belt

Lithosphere

Asthenosphere

Shield

Stableplatform

Mantle Mantle

FIGURE 1.6 The outermost layers of the solid Earth,based on physical characteristics, are the asthenosphere and the lithosphere.Theasthenosphere is hot, close to its melting point, and is capable of plastic flow.The lithosphere abo ve it is cooler and rigid. It includes the uppermost p

of the mantle and two t ypes of crust: thin, dense oceanic crust and thick, buoyant continental crust.

What are the fundamental structurafeatures of continents?


15/26

16 C h a p t e r 1

100400

8

4

0

4

8Elevation(km)

Mount Everest(8.8 km)

Mean land surface(0.84 km)

Mean seafloor(3.7 km)

Mariana Trench(11.0 km)

Percent of Earths surface20 60 80

FIGURE 1.7 A graph of the elevation of the continents and ocean basinsshows that theaverage height of t he continents is only 0.8 km above sea level. Only a small percentage of Ea rths

surface rises above the avera ge elevation of the continents or drops below t he average elevation of

the ocean floor.

of the ocean. If the continents did not rise quite so high above the ocean floor, the

entire surface of E arth wo uld be covered with wa ter.

Shields. The extensive flat region of a continent, in which complexly deformed

ancient crystalline rocks are exposed, is known a s a shield(Figure 1.8).A ll of therocks in the shield formed long agomost more than 1 billion years ago.

Moreover, these regions have been relatively undisturbed for more tha n a ha lf-

billion yea rs except for b road, gentle warping.The rocks of the shields are highly

deformed igneous and meta morphic rock; they are also called the basementcomplex.

Without some firsthand knowledge of a shield, visualizing the nature and sig-

nificance of this important pa rt of the continental crust is difficult. Figure 1.9

shows part of the Canadian shield of the North American continent as seen

from space. It w ill help you to comprehend the extent, the complexity, and some

of the typical features of shields. First, a shield is a regional surface of low relief that

generally has an elevation within a few hundred meters of sea level. (Reliefis theelevation difference betw een the low and the high spots.) Resistant rocks may rise

50 to 100 m above their surroundings.

A second characteristic of shields is their complex internal structure and com-plex arrangements of rock types. Many rock bodies in a shield once were molten, and

others have been compressed and extensively deformed w hile still solid. Much of the

rock in shields was formed severa l kilometers below t he surface. They are now ex-

posed only beca use the shields have been subjected to extensive uplift and erosion.

Stabl e Pl atforms. When the basement complex is covered with a veneer of

sedimentary rocks, a stable platformis created. The layered sedimentary rocks arenearly horizonta l and commonly etched by dendritic (treelike) river patterns (Figure

1.10). These broad a reas ha ve been relat ively stable throughout the la st 500 million

or 600 million years; that is, they have not been uplifted a grea t distance above sea level

or submerged far below ithence the term stable platform. In North America, the

stable platform lies between the Appalachian M ountains and the R ocky Mountains

and extends northward to the Lake Superior region and into western Canada.Throughout most of this area, the sedimentary rocks are nearly horizontal, but locally

they have been w arped into broa d do mes and ba sins (Figure 1.8). Sometimes it is

useful to group the shield and stab le platf orm together in wha t is called a craton.

Folded M ountains. Some of t he most impressive features of the continents are

the young folded mountain belts that t ypically occur along their margins. Mostpeople think of a mounta in as simply a high, rugged landform, standing in contra st

to flat plains and lowlands. Mounta ins, however, are much more than high country.

To a geologist, the term mountain belt means a long, linear zone in E arths crust

where the rocks have been intensely defo rmed by horizontal stress during the slow

What are the most significant featuresof continental shields?

What are the most significant featuresof stable platforms?


16/26

Young mountain belt0 to 100 m.y. old

Older mountain belt100 to 500 m.y. old

Stable platformless than 500 m.y. old

Shieldgreater than 500 m.y. old

Flood basaltless than 200 m.y. old

Continental shelf

Oceanic crust0 to 200 m.y. old

Oceanic ridge

Trench

FIGURE 1.8 The major surface features of Earthreflect thestructure of the lithosphere. The continenta l crust rises abo ve the

ocean ba sins and fo rms continents. They have a s their major

structural features shields, stable platforms,a nd folded mounta in

belts. The continents are fo rmed mostly o f granitic rock. The oceanic

crust forms the ocean f loor. Its major features include the oceanic

ridge, the abyssal floor, seamounts, and trenches. It is composed

primarily of basalt.



17/26

collision between two l ithospher ic pla tes. In addit ion, they genera l ly have

been intruded by molten rock. The topography can be high and rugged, or it

can be w orn down to a surface of low relief.To a sightseer, the topography ofa mountain belt is everything, but to a geologist , it is not as important a s the

extent and style of its internal deforma tion. The great folds and fractures in

mountain belts provide evidence that E arths lithosphere is, and ha s been, in

motion.

Figure 1.10 illustra tes some chara cteristics of folded mounta ins and the ex-

tent to which the margins of continents have been deformed. On this map of

the Appalachian Mounta ins, the once horizontal layers of rock have been de-

formed by compression and are folded like wrinkles in a rug. E rosion has re-

moved the upper parts of the fo lds, so the resistant layers form zigzag patterns

similar to those that would be produced if the crest of the wrinkles in a rug

were sheared off.

The crusts of the Moon, Mars, and M ercury lack this type of deformation.A ll

of their impact craters, regardless of age, are circularproof that the crusts ofthese planets have not been strongly deformed by compressive forces. Their

crusts, unlike that of E arth, appear to have b een fixed and immovable through-

out their histories. H owever, Venus is like E arth in this respect and has long belts

of folded mountains.

Summar y of the Conti nents. The broad, flat continental masses that rise above

the ocean ba sins have an a lmost endless variety of hills and valleys, plains and

plateaus, and mountains. Yet from a regional perspective,t he geologic differences

betw een continents are mostly in size and shape a nd in the proportions of shields,

stable platforms, and fo lded mountain belts.

Can a folded mountain belt be alowland?

18 C h a p t e r 1

FIGURE 1.9 The Canadian shieldis a fundamental structural component of Nort h America. It is composed of complexly deformed crystallinerock bodies, eroded to a n almost flat surface near sea level, as shown in this false-color satellite image. Throughout much of the Ca nadian shield, the

topsoil has been removed by glaciers, and d ifferent rock bodies are etched in relief by ero sion.The resulting depressions commonly are filled with

water, forming lakes and bogs that emphasize the structure of the rock bodies. D ark tones show area s of metamorphic rock. Light pink tones show

areas of gra nitic rock. (Courtesy of NA SA )


18/26


Let us now b riefly review the ma jor structural components of the continents by

examining North a nd South A merica (Figure 1.8). North America has a large

shield, most of w hich is in Cana da. Most of the C anadia n shield is less than 300 m

ab ove sea level. The rocks in the Ca nad ian shield formed betw een 1 and 4 billion

years ago. The stable platform extends through the central U nited Stat es and west-

ern Ca nada and is underlain by sedimentary rocks, slightly warped into broad

domes and basins. The Appalachians are an o ld folded mountain belt that formed

about 250 million years a go. The R ocky Mountains form part of a nother folded

mountain belt (the Cordillera) that dominates western North America and ex-

tends into South America. The R ockies started fo rming about 60 million years

ago, and pa rts of this belt are still active.

In ma ny wa ys, the structure of South America (Figure 1.8) is similar to t hat o fNorth America. The continent consists of a bro ad shield in B razil and Venezuela,

and stab le platforms in the Amazon ba sin and along the eastern flanks of the

And es Mountains. The And es Mountains are part of the C ordilleran folded moun-

tain belt t hat extends from Alaska to the southern tip of South America. The con-

tinent has no mounta in belt along the eastern margin like the Appa lachian Moun-

tains in North America. More tha n 90% of South America drains into the Atlantic

Ocean by w ay of t he Ama zon R iver system.

B efore going on, you should briefly review the major structural features of each

of the other continents and examine how they a re similar a nd how they are dif-

ferent (see the shaded relief ma p inside the ba ck cover).

FIGURE 1.10 The stable platform and folded mountain belt in the eastern United Statesare clearly shown o n this topographic map. Thelayered rocks of the stable platform are nearly horizontal, but on a regional scale, they are wa rped into a large structural basin that has been

highly dissected by intricate netw orks of river valleys (upper left). The rocks of the Appalachian Mo untains, in contrast, have been compressed

and fo lded (center and lower right). The folded layers are expressed by long, narrow ridges of resistant sandstone that rise about 300 m above

the surrounding area (lower right). E rosion has removed the upper parts of many folds, so their resistant limbs (which form the ridges) are

exposed in elliptical or zigzag outcrop pa tterns. (Cour tesy of K en Perry, Chalk B utte, I nc.)


19/26

Topogra phic maps that show elevat ions have alwa ys been

important to geologists studying the continents. U ntil the

mid-1900s, such maps were painstakingly constr ucted by

careful fieldwork using survey instruments. It might take

weeks to cover an area of 100 km2, assuming access was

good . La ter in the 1930s and 1940s, aeria l photographs re-placed or supplemented these field techniques, but some

fieldwork w as still necessary. H owever, many remote a reas

and less-developed countries remained la rgely unmapped.

In Februa ry 2000, astrona uts on the space shuttle using

imaging rada r revolutionized mapmaking. In just nine days,

they collected the data for the most accurate topographic

map ever made of much of the planet. R ada r signals were

bounced off E arths surface and t hen received by two d if-

ferent antennas, one inside the spacecraft a nd the other on

a 60-m-long boo m extended fro m the shuttle. A com puter

then combined these separate ima ges to prepare a three-di-

mensional topogra phic map, just as your brain combines

two separate images, one from ea ch eye,t o construct a 3-Dimage of your surroundings.

A key advantage to radar is that it can see the sur-

face through clouds and in da rkness. Ano ther major ad-

vantage is speed. Shuttle radar can capture the topograph-

ic data for an area the equivalent of Rhod e Island in only

two seconds and for an area 100,000 km2 in a minute. In

nine days, the shuttle mapped nearly 80% of E arths land

surface. In many a reas these will be the highest resolution

maps available. B efore the shuttle mission, less than 5% of

E arths surface had been mapped at a comparab le scale.

NASA, the U.S. D epartment of D efense, and the G er-

man a nd Ita lian space agencies are supporting the project.

The most det ailed ma ps, showing ob jects just 3 m acro ss,

may only be available to the U.S. military. Such data will be

used, for example, to guide cruise missiles through complex

terrains and assist in troop deployment. Low er resolution

maps (10 to 30 m resolution) will be used to study E arth on

a globa l scale in a way never before possible, including top-

ics such as tectonics, flooding, erosion rates, volcanic and

landslide hazards, earthquakes, and climate change.

The shuttle radar topographic map below shows the dra-

matic difference between the new data (30 m resolution on

the left) and the best existing data (on the right) for the trop-

ical rain forests of central B razil. This region is centered near

the city of Ma naus on the great Ama zon R iver. With the new

map, you can see the delicate branching patterns of a multi-tude of stream valleys. The dark, smooth area s are reservoirs

behind large dams. Most of the small valleys are not visible

on the earlier topographic map.

STATE OF

THE ARTM appi ng the Cont i nents from Space

20

(Courtesy of NA SA)

0 10 20

km

0 20

km

10


20/26


MAJ OR FEATURES OF THE OCEAN BASINS

O ceanic crust differs strikingly from continenta l crust in rock types,

structure, landforms, age, and origin.The major features of the ocean floor

are (1) the oceanic ridge, (2) the aby ssal floor, (3) seamounts, (4) trenches,

and (5) continenta l margins.

The ocean floor, not the continents, is the typical surface of the solid E arth. Ifwe could dra in the oceans completely, this fact wo uld be ob vious. The seafloor

holds the key to the evolutio n of E art hs crust, but not unt il the 1960s did we rec-

ognize that fact a nd obta in enough seafloor da ta to see clearly its regional char-

acteristics. This new knowledge caused a revolution in geologists ideas abo ut

the nature and evolution of the crust, a revolution as profound as D arwins the-

ory of evolution.

U ntil about t he 1940s, most geologists believed that the ocea n floor wa s simply

a submerged version of the continents, with huge areas of flat a byssal plains cov-

ered with sediment eroded from the land mass. Since then, great ad vances in tech-

nology and exploration have been used to map t he ocean ba sins in remarkable

detail, as clearly as if the water ha d been removed (see the inside cover of this

book). These maps show tha t submarine topography is as varied as that o f the con-tinents and in some respects is more spectacular.

Along w ith this kind of ma pping, we ha ve collected samples of the oceanic crust

with drill rigs, dredges, and subma rines. We have learned tha t the oceanic crust is

mostly basalt, a dense volcanic rock, and tha t its major topographic features are

somehow rela ted to volcanic activity.These features make the oceanic crust entirely

different from the continental crust. Moreover, the rocks of the ocean floor are

young, in geologic terms. Most are few er than 150 million years old, whereas the

ancient rocks of t he continenta l shields are mo re tha n 600 million years old. We

have discovered that the rocks of the ocean floor have not been deformed into

folded mounta in beltsin marked contrast to the complexly deformed rocks in the

mountains and basement complex of the continents.

The Oceani c Ri dge. The oceanic ridgeis perhaps the most striking and import antfeature on the ocean floor. It extends continuously from the Arctic B asin, down the

center of the Atlantic Ocean, into the Indian Ocean, and across the South Pacific.

You can see it clearly in Figure 1.8 and on the ma p inside the b ack cover. The

oceanic ridge is essentially a bro ad, fractured rise, generally more than 1400 km

wide. It s higher peaks rise as much as 3000 m ab ove their surroundings. A huge,

cracklike rift valleyruns along the axis of the ridge thro ughout much of its length,which totals about 70,000 km. In ad dition, great fracture systems, some as long

as 4000 km, trend perpendicular to the ridge.

The A byssal Fl oor. The oceanic ridge divides the Atlantic and Indian oceans

roughly in half and tra verses the southern and eastern parts of the Pa cific. On bo th

sides of the ridge are vast areas of broa d, relatively smooth deep-ocean ba sins

known as the abyssal floor.This surface extends from the flanks of the oceanicridge to the continental ma rgins and generally lies at d epths of a bout 4000 m.

The abyssal floor can be subdivided into tw o sections: the aby ssal hills and t he

ab yssal plains. The abyssal hillsare rela tively small ridges or hills, rising as muchas 900 m abo ve the surrounding o cean f loor. They cover f rom 80% to 85% of the

seafloor, and thus, they are the most widespread landforms on Earth. Near the

continenta l margins, land -derived sediment completely covers the aby ssal hills,

forming flat, smooth abyssal plains.

Trenches. The deep-sea trenchesare the lowest areas on E arths surface. TheMariana Trench, in the Pa cific Ocean, is the deepest part of t he worlds oceans

What are the highest and lowest partsof the ocean floor?

How do the landforms on the oceanfloor differ from those on continents?


21/26

11,000 m below sea leveland ma ny ot her trenches are more tha n 8000 m deep.

Trenches have at tracted t he attent ion of geologists for years, not only because of

their depth, but also because they represent fundamental structural features of

E arths crust. As illustrat ed in Figure 1.11, the trenches are invariab ly adjacent tochains of volcanoes called island arcs or to coastal mountain ranges of thecontinents.Why? We will see in subsequent chapters how the trenches a re involved

in the planets most intense volcanic and seismic (earthqua ke) activity, and how the

movement of E arth s lithospheric plat es causes it all.

Seamounts. Isolated peaks of submarine volcanoes are known as seamounts.Some seamounts rise ab ove sea level and form islands, but most a re submerged and

are known o nly from oceanographic soundings. Although many ma y seem to occur

at random, most, such as the Haw aiian Islands, form chains along well-defined

lines. Islands and seamounts testify to the extensive volcanic activity that is ongoing

throughout the ocean basins.They also provide important insight into the dynamics

of the inner E arth.

Continental M argins. The zone of transition betw een a continent and a n ocean

basin is a continental margin. The submerged part of a continent is called acontinental shelf, essentially a shallow sea that extends around a continent formany kilometers. You can clearly see the continenta l shelf around t he continents

in Figure 1.8 and on the ma p inside the back cover. G eologically, the continental

shelf is part of the continent, not part of the ocean basin. At present, continental

shelves form 11% of the continental surface, but a t times in the geologic past, these

shallow sea s were much more extensive.

The seafloor descends in a long, continuous slope from the outer edge o f the con-

tinental shelf to the deep-ocean basin.This continental slopemarks the edge of the

How are continental margins differentfrom the rest of the seafloor?

22 C h a p t e r 1

Oceanic ridge Trench Seamounts Abyssal floor Continental Shelf

FIGURE 1.11 The major features of the ocean floor include the oceanic ridge, the deep-sea trenches, and the a byssal floor. Seamounts riseabove t he deep-ocean floor and a re formed by vo lcanic eruptions.


22/26


E arths uncharted f rontiers lie at t he floor of the oceans,

and f or most of human history they w ere as inaccessible as

the stars. Thanks to several new techniques, geologists are

now seeing the topography of the ocean floor.The technique that is easiest to understand involves echo

sounding. This is a special type of sonarsound waves are

timed while they travel to the ocean bottom and bounce

back. R esearch ships tow long trails of hydrophones be-

hind them to detect the signals. The result is a narrow strip

map showing the elevation of the seafloor directly beneath

the ship. Multiple traverses are necessary to a ccumulate

enough data to compile a good topographic map. It would

take a bout 125 years to ma p all of the ocea n basins using this

method.

A completely new way to make global maps of the

seafloor is carried out by a n orbiting spacecraft instead of

a ship. The satellites use rada r to carefully map the eleva-tion of the sea surfa ce.These maps show t hat t he surface of

the ocean bulges upward and dow nward, mimicking the

topography of t he underlying ocean floor. Although these

bumps are too small to be seen with your eyes, they can be

measured by a ra da r altimeter on the satellite.The satellite

emits a pulse of rada r at the ocean surface, and the time

for its reflection back to the satellite is measured.The width

of the pulse is several kilometers wide and averages out

local irregularities caused by ocea n wa ves. To make a ccu-

rate elevation measurements, the satellite itself is tracked

from ground stations using lasers.

The dat a a re then processed with a computer to calculate

the topogra phy of the underlying seafloor. The maps pro-

vide the first view of the o cean-floor structures in many r

mote areas of the E arth. The map shown here of a swa

across the Pa cific Ocean with New Z ealand on the wesand the map on the inside back cover were constructed

this wa y.

Why does the sea surface bulge? I t bulges because Ea rth

gravitationa l field is not constant everywhere. The gravit

tional acceleration at a ny spot on Ea rths surface is propo

tional to the mass that lies directly beneath it. Thus, if a hig

seamount or ridge lies on the ocean floor, it has enough ma

to pull water toward it, piling up the water immediate

above it. Such a bulge may be several meters high. On th

other hand, because water has a density less than that

rock, a point ab ove a deep trough in the ocean floor has le

mass directly below it a nd shows up as a shallow trough o

the sea surface.

STATE OF

THE ARTM appi ng the Ocean Floor f rom Space

2

(Courtesy of D. T. Sandwell, Scrip ps I nstitution of Oceanography,U niversity of Calif orni a at San Di ego)

Seasurface

Seafloor

Seamount OCEAN

CRUST

SatelliteOrbit

Radarpulse

Trackinglaser

Antenna


23/26

continental rock mass. Co ntinental slopes are found around the margins of every

continent and around smaller fragments of continental crust, such as M ada gascar

and New Z ealand. Look a t Figure 1.8 and study the continental slopes, especially

those surrounding North America, South America, and Africa. You can see that

they form one of E arths major topographic features. On a regional scale, they are

by fa r the longest and highest slopes on E arth. Within this zone, from 20 to 40 km

wide, the average relief abo ve the seafloor is 4000 m.I n the trenches that run a long

the edges of some cont inents, relief on the continenta l slope is as great a s 10,000

m. In contrast to t he shorelines of the continents, the continental slopes are re-

markably straight over distances of thousands of kilometers.

To ensure that you understand t he basic features of the oceans, refer aga in to

Figure 1.11 and the ma p inside the b ack cover a nd study t he regional relationships

of the o ceanic ridges, abyssal plains, trenches, and seamounts of ea ch of the ma jor

oceans. For example, the topography of the Atlantic Ocean floor shows remark-

ab le symmetry in the distribution of the major fea tures (Figure 1.11). It is domi-

nated by the Mid-Atlantic R idge, a broa d rise in the center of the basin. Iceland

is a part of the Mid-Atlantic Ridge that reaches above sea level. South of Iceland,

the ridge separa tes the ocean floor into two long, para llel sub-ba sins that a re cut

by fra cture zones stretching across the entire basin. Ab yssal hills lie on either

side the ridge, and abyssal plains occur along the ma rgins of t he continental plat-

forms. In the South Atlantic, two symmetrical chains of seamounts extend from thecontinental margins to the oceanic ridge and come together to form an open V.

D eep trenches flank volcanic island a rcs off the nort h and south margins of South

America. The symmetry of the At lantic B asin even extends to the continenta l

margins: the outlines of Africa and E urope fit those of South America and No rth

America.

THE ECOSPHEREA MODEL OF PLANET EARTH

One w ay to simplify this vast array of d etails is to consider a mod el of E arth

in its simplest fo rm, in which the funda mental componentsenergy, rock,

air, wa ter, and lifeare the only elements.

H aving surveyed the important components of P lanet Ea rth, you should now look

ba ck and consider wha t these facts imply for life in general and humans in partic-

ular. To help you understand, let us contemplate a simple model of E arththe

ecosphere.

An ecosphere is a small glass globe, about the size of a large canta loupe, con-taining five essential elements: energy, air, water, sand, and living things (algae,

seaweed, shrimp, snails, and a variety of microorganisms). The globe is sealed,

forming a closed system in which plants and animals are self-sustaining (Figure

1.12). Just like a planet, nothing enters or leaves the system except sunlight and

heat. You cannot ad d oxygen.You can never clean the wa ter or replace the seaweed

or remove dead organisms. You can never add fo od o r remove waste. The plants

and a nimals are on their own small planet: an isolated world in miniature.E xperiments have shown tha t if even one of the five parts is missing, the shrimp

will not survive and t he entire system will fail. The biological cycle is shown in the

diagra m in Figure 1.12.The key to the system is energy in the form o f light. Light

energy powers photosynthesis, the chemical mechanism through which algae make

their own food f rom carbon dioxide and wa ter and release oxygen into the wa ter.

The shrimp breathe the oxygen in the water and feed on the algae and bacteria.

The bacteria b reak dow n the anima l waste into nutrients that the algae use in their

growth. The shrimp, snails, and bacteria also give off carbon dioxide, which the

alga e use to produce oxygen.Thus, the cycle is repeated and co nstantly renews it-

self. The shrimp and snails are ma sters of this little worldas long a s they do no t

24 C h a p t e r 1


24/26


overpopulate or contaminate their environment. In this closed system, plants

and animals grow, reproduce, and die, but the self-renewing cycle continues. The

ecosphere in Figure 1.12 operat ed well for more tha n three years, until it was

moved to a spot near a w indow. There it received more sunlight. The algae grew

too fast a nd upset the critical ba lance, causing everything to die. Some ecospheres

have sustained t hemselves for mo re than 10 years.

As you might suspect, an ecosphere is much the same a s Planet E artha closed,

self-conta ined system with a few b asic parts. The only real external input is ener-

gy from sunlight. Our ecospherethe lithosphere, atmosphere, hydrosphere, andbiosphereprovides the rest.When astrona uts look ba ck at our planet from space,

they see an ecosphere made of continents and oceans, forests, and polar bodies of

ice, all enclosed in a thin blue dome of gases, bathed in sunlight.

These five spheres interact to fo rm a single dynamic system in which com-

ponents are interconnected in fascinating ways with a strength so amazing that a

change in one sphere can aff ect the others in unsuspected wa ys. In the next chap-

ter, we will consider in greater deta il the concept of nat ural systems and introd uce

you to the two fundamental geologic systems of our planetary ecosphere.

Organicwaste

Bacteria

Inorganicnutrients

Algae

Sand

Carbon dioxide

Food and oxygen

Sea weed

Water

Solar energy

Air

Shrimp and snails

FIGURE 1.12 An ecosphere is a small model of E arth. It is a closed system of air, water, sand,and living organisms. Sunlight enters, providing the energy needed by the algae to make oxygen

from carbo n dioxide and wat er. Shrimp and snails breathe the oxygen and consume some algae and

bacteria. B acteria break down the animal waste into nutrients, which are used by the algae.The

shrimp, snails, and bacteria also give off carbon dioxide, which the algae use to make more oxygen.

This is an interlocking cycle of constant decay a nd renewal. If a ny component is missing, or is too fa rout of proportion, the system collapses and the entire biosphere of t his tiny planet becomes extinct.

(P hotograph by Stan Macbean)


25/26

26 C h a p t e r 1S l o p e S y s t e m s

26

3. D etailed chemical studies show tha t these metallic mete-

orites are made principally of only tw o elements, iron and

nickel.4. Iron meteorites are also extremely dense, a cubic cen-

timeter w eighs nearly 8 g/cm3, compared to a typical sur-

fa ce rock tha t weighs less than 3 g/cm3.

Interpretations

Each of these bits of evidence points to a logical interpre-

tat ion that ha s implications for the nature of E arths interi-

or. Appa rently, iron meteorites formed by: (1) partia l melt-

ing of a planet (implied by high temperature of f ormat ion);

(2) gravitationa l sinking of the molten meta l to nea r its cen-

ter (high density a nd slow cooling rate ca used by a t hick in-

sulating layer that allowed heat to escape slowly), and fi-nally (3) cooling and crystallization (crystalline structure).

Thus, we have concluded that when you hold a n iron mete-

orite in your hand, you are actually holding a piece of the

once molten core of a nother planet. The other types of me-

teorites appear to ha ve come from the ma ntles and crusts of

small planets tha t were like Ea rth in having differentiated in-

teriors.

The average density of the E art h is 5.5 g/cm3. This aver-

age density of the surface rocks is about 2.8 g/cm3. How does

this support the interpretation a bove?

GeoLogic Meteorites and Earths Interior

Stony meteorite Mantle and crust

(seen through microscope)

Stony-iron meteorite Core-mantle boundary

Iron meteorite Core

(Photograph of stony-iron meteorite by C. Clark, Smithsonian Institution)

Can you imagine any part of Earth more inaccessible than

the core? Our best estimates place it at a depth of almost

3000 km.The deepest mines or even caves extend only a fewkilometers deepin spite of wha t you may ha ve read in Jules

Vernes Jour ney to the Center o f the Earth. Our deepest

drill holes only penetrate to a depth of 15 or 20 km. And

yet geologists believe they know a great deal about Earths

interior.For example they believe that E arths core is made

mostly of iron, that it supports a huge magnetic field, and

that it ha s a temperature of 5000 C . While we cannot

justify all of these propositions here, we can present a few o f

the facts that lead to the logical conclusion that E arths core

is made of molten metal.

Observations

1. There are three fundament ally different types of mete-

orites (rocks that fa ll from space). The most common a re

similar to rocks found at E arths surface and are called

stony meteorites. Ot her meteorites are mixtures of stony

materials and shiny metal, and a third type is made sole-

ly of metal.

2. When cut open, polished and etched with acid, metallic

meteorites have spectacular crystalline structures that re-

veal they formed a t high temperatures from molten metal

and then crystallized slowly in the solid state.


26/26


KEY TERMS

aby ssal floor (p. 21)

aby ssal hill (p. 21)

aby ssal plain (p. 21)

accretionary heat (p. 9)

asthenosphere (p. 14)

atmo sphere (p. 10)

basement complex (p. 16)

biosphere (p. 11)

continent (p. 13)

continental crust (p. 15)

continental ma rgin (p. 22)

continental shelf (p. 22)

continental slope (p. 22)

core (p. 13)

craton (p. 16)

crust (p. 13)

density (p. 5)

differentiat ed planet (p. 13)

ecosphere (p. 24)

folded mounta in belt (p. 16)

geology (p. 4)

hydrosphere (p. 11)

impact crater (p. 9)

inner core (p. 14)

inner planets (p. 5)

internal heat (p. 9)

island arc (p. 22)

lithosphere (p. 14)

mantle (p. 13)

mesosphere (p. 14)

ocean ba sin (p. 13)

oceanic crust (p. 15)

oceanic ridge (p. 21)

outer core (p. 14)

outer planets (p. 5)

radioa ctivity (p. 9)

relief (p. 16)

rift valley (p. 21)

seamount (p. 22)

shield (p. 16)

stable platform (p. 16)

trench (p. 21)

REVIEW QUESTIONS

1. Why are some planets geologically active today and othersinactive?

2. Why are the a tmosphere and the o ceans considered a smuch a part of E arth a s is solid rock?

3. Study the view of E arth in Figure 1.3. Sketch a map show-ing: (a) ma jor patterns of atmospheric circulation,(b) low-

latitude deserts, (c) the tropical belt, (d) a folded mountain

belt, (e) the stable platform, and (f) the shield of North

America.

4. D raw two d iagrams of E arths internal structure. D raw oneto show its internal structure based o n chemical composi-

tion and dra w ano ther showing its structure based on mechan-

ical (physical) propert ies.

5. D raw a cross section showing the lithospheres major struc-tural features: the continental crust, shields, stable plat-

forms, and folded mountain belts, together with the oceanic

ridge, the abyssal floor, and deep-sea trenches.

6. Make a ta ble comparing the differences in the age, thick-ness, density, composition, and structure of oceanic and co

tinental crust.

7. B riefly describe the distinguishing feat ures of continentalshields, stable platf orms, and folded mounta in belts.

8. U sing the map in the back of the book, describe the loca-tions of the shield, stable platforms, and folded mountain

belts of Asia, Africa, Australia, and E urope.

9. B riefly describe the distinguishing feat ures of the oceanicridge, the abyssal floor, trenches, seamounts, and continent

margins.

10. D escribe the major elements of an ecosphere and how itfunctions. R elate these elements to their counterparts on

the real Earth.

ADDITIONAL READINGS

Allegre, C. 1992. From Stone to Star. Cambridge,Ma ss. : Harvard

U niversity P ress.

B roecker, W. S. 1998. H ow to Build a H abitable Planet. New

York, N.Y.: Columbia U niversity P ress.

Lunine, J. I. 1999.Earth: Evolution of a H abitable World. Cam-

bridge, U.K.: Ca mbridge University Press.

Sagan, C. 1994. Pale Bl ue D ot. New York: Ra ndom H ouse.

Hughes, R . 2002. Planet Earth. New York: Alfred A . Knopf.

Ear ths D ynami c Systems Websi te

The compa nion Website a t www.prenhall.com/hamblinprovides you with an on-line study guide and a ddition-

al resources for each chapter, including:

On-line Quizzes (Chapter Review, Visualizing G eology,

Quick Review,Vocabulary Flash Cards) with instant feedback

Quantita t ive Problems

Critical Thinking Exercises

Web Resources

Ear ths Dynami c Systems CD

Examine the CD that came with your text.It is design

to help you visualize and t hus understand the concep

in this chapter. It includes:

Animations of complex global systems

Video clips of Earth systems in action

Slide shows with examples of important geologic features

A direct link to the Companion Web Site

MULTIMEDIA TOOLS

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